To say it another way, the points you want to use must be valid for the dates of the cruise. What use year (2003 or 2004) does your desired cruise fall into?
Let's say your cruise is in April, 2004. You need to figure out which use year contains April, 2004. It could be your 2003 use year or your 2004 use year.
For example, if you have a February use year, a 4/04 cruise would fall within your 2004 use year (which is 2/1/2004 - 1/31/2005). If you have an August use year, an April ,2004 cruise would fall within your 2003 use year (which is 8/1/2003 - 7/31/2004). You can do the same type of "figuring" for your specific situation to see which use year "contains" the dates of your cruise.
Once you know "the use year of your desired cruise", you can determine whether you will need to bank or borrow points to pay for it. You can always book/reserve 11 months ahead of departure when you use points.